Flowering Plants Could Be Making Tropics Cooler

The world is a cooler, wetter place because of flowering plants, according to new climate simulation results published in the journal Proceedings of the Royal Society B. The effect is especially pronounced in the Amazon basin, where replacing flowering plants with non-flowering varieties would result in an 80 percent decrease in the area covered by ever-wet rainforest.

Flowering Plants Could Be Making Tropics Cooler

The simulations demonstrate the importance of flowering-plant physiology to climate regulation in ever-wet rainforest, regions where the dry season is short or non-existent, and where biodiversity is greatest.

"The vein density of leaves within the flowering plants is much, much higher than all other plants," said the study's lead author, C. Kevin Boyce, Associate Professor in Geophysical Sciences at the University of Chicago. "That actually matters physiologically for both taking in carbon dioxide from the atmosphere for photosynthesis and also the loss of water, which is transpiration. The two necessarily go together. You can't take in CO2 without losing water."

This higher vein density in the leaves means that flowering plants are highly efficient at transpiring water from the soil back into the sky, where it can return to Earth as rain.

"That whole recycling process is dependent upon transpiration, and transpiration would have been much, much lower in the absence of flowering plants," Boyce said. "We can know that because no leaves throughout the fossil record approach the vein densities seen in flowering plant leaves."

For most of biological history there were no flowering plants  known scientifically as angiosperms. They evolved about 120 million years ago, during the Cretaceous Period, and took another 20 million years to become prevalent. Flowering species were latecomers to the world of vascular plants, a group that includes ferns, club mosses and confers. But angiosperms now enjoy a position of world domination among plants.

"They're basically everywhere and everything, unless you're talking about high altitudes and very high latitudes," Boyce said.

Dinosaurs walked the Earth when flowering plants evolved, and various studies have attempted to link the dinosaurs' extinction or at least their evolutionary paths to flowering plant evolution. "Those efforts are always very fuzzy, and none have gained much traction," Boyce said.

Boyce and Lee are, nevertheless, working toward simulating the climatic impact of flowering plant evolution in the prehistoric world. But simulating the Cretaceous Earth would be a complex undertaking because the planet was warmer, the continents sat in different alignments and carbon- dioxide concentrations were different.

"The world now is really very different from the world 120 million years ago," Boyce said.

Building the Supercomputer Simulation

So as a first step, Boyce and co-author with Jung-Eun Lee, Postdoctoral Scholar in Geophysical Sciences at UChicago, examined the role of flowering plants in the modern world. Lee, an atmospheric scientist, adapted the National Center for Atmospheric Research Community Climate Model for the study.

Driven by more than one million lines of code, the simulations computed air motion over the entire globe at a resolution of 300 square kilometers (approximately 116 square miles). Lee ran the simulations on a supercomputer at the National Energy Research Scientific Computing Center in Berkeley, Calif.

"The motion of air is dependent on temperature distribution, and the temperature distribution is dependent on how heat is distributed," Lee said. "Evapo-transpiration is very important to solve this equation. That's why we have plants in the model."

The simulations showed the importance of flowering plants to water recycling. Rain falls, plants drink it up and pass most of it out of their leaves and back into the sky.

In the simulations, replacing flowering plants with non-flowering plants in eastern North America reduced rainfall by up to 40 percent. The same replacement in the Amazon basin delayed onset of the monsoon from Oct. 26 to Jan. 10.

"Rainforest deforestation has long been shown to have a somewhat similar effect," Boyce said. Transpiration drops along with loss of rainforest, "and you actually lose rainfall because of it."

Studies in recent decades have suggested a link between the diversity of organisms of all types, flowering plants included, to the abundance or rainfall and the vastness of tropical forests. Flowering plants, it seems, foster and perpetuate their own diversity, and simultaneously bolster the diversity of animals and other plants generally. Indeed, multiple lineages of plants and animals flourished shortly after flowering plants began dominating tropical ecosystems.

The climate-altering physiology of flowering plants might partly explain this phenomenon, Boyce said. "There would have been rainforests before flowering plants existed, but they would have been much smaller," he said.

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